This invention relates to an exhaust system for a marine propulsion unit and more particularly to an improved low speed exhaust gas discharge for such a unit and also to an arrangement for insuring cooling of the exhaust gases without heating the other fluids of the engine adversely.
It is well known in marine engines to silence the exhaust gases emanating from the engine, be at either an inboard or an outboard mounted engine, by passing them through the body of water in which the watercraft is operated. This is normally accomplished by a form of underwater exhaust gas discharge, such as those that discharge the exhaust gases through the hub of the propeller. However, when the watercraft is operating at a low speed, the underwater exhaust gas discharge will become relatively deeply submerged and this coupled with the relatively low exhaust gas pressure can give rise to high back pressure in the exhaust system. For that reason, it is the normal practice to employ a further, low speed, above the water exhaust gas discharge for discharging the exhaust gases directly to the atmosphere without flowing through the body of water in which the watercraft is operating under this running condition.
Normally the above water exhaust gas discharge is generally open under all conditions and the flow path to it is provided with restrictions so as to ensure against excess noise when operating under high speed conditions. However, there are times when the exhaust pressure may, in the exhaust system, become negative. This means that air and entrained water may enter the exhaust system through the exposed above the water exhaust gas discharge. The problems attendant with such a result should be obvious.
It is, therefore, a principal object of this invention to provide an improved above the water exhaust gas discharge system for a marine engine wherein water entry into the exhaust system will be precluded.
In conjunction with some forms of marine engines such as outboard motors, a four cycle internal combustion engine is employed which has a lubricant reservoir that depends into the drive shaft housing of the outboard motor. Also, the drive shaft housing is employed as an expansion chamber for silencing of the exhaust gases. In order to permit a compact arrangement it has been the practice to surround at least a portion of the exhaust system with the oil reservoir. However, in order to preclude against undue heating of the lubricant in the oil reservoir it has been the practice to provide a cooling jacket between the exhaust pipe exterior and the interior of the surrounding lubricant reservoir. However, for strength and manufacturing purposes it is desirable to provide some mechanical interconnection between the exhaust pipe and the interior of the lubricant reservoir, such as interconnecting ribs. These ribs obviously will transmit heat from the exhaust pipe to the lubricant reservoir.
It is, therefore, a still further object of this invention to provide an improved lubricant reservoir and exhaust pipe system wherein the exhaust pipe can be supported by the lubricant reservoir but wherein the exhaust pipe heat will not be transmitted significantly to the lubricant in the lubricant reservoir.